"The teams that will be going in there have received extensive training to deal with these sorts of incidents," he told Sky News.
Valerio said he hoped negotiators would be able to subdue the situation.
"But this could be replicated across the estate very quickly," he said.
The trade union official said there are no prison officers actually located on the rioting unit, which is separated from the rest of the prison.
A decision would have to be made whether prison guards go into the wing to try to subdue it, he said.
Maidstone, with an inmate population of about 600, is a category C training prison that predominantly houses sex offenders from the Kent and Sussex areas.
Its website says it aims to "create a therapeutic environment that supports, embraces and empowers change with a primary focus on risk reduction and protection".
The prison also takes in a small number of foreign prisoners with more than 18 months to serve and provides forums with the UK Border Agency.
Kent Police confirmed they were aware of the incident, but a spokeswoman said prison services were dealing with the matter.
-AAP
